1. Field of the Invention
The present invention relates to an apparatus with a plurality of charged-particle beams. More particularly, it relates to an apparatus which employs plural charged-particle beams to simultaneously acquire images of plural scanned regions within an observed area on a sample surface. Hence, the apparatus can be used to inspect and/or review defects on wafers/masks with high resolution and high throughput in semiconductor manufacturing industry.
2. Description of the Prior Art
The following description and examples are not admitted to be prior art by their mention in this Background section.
For manufacturing semiconductor IC chips, pattern defects and/or uninvited particles (residuals) inevitably appear on surfaces of wafers/mask during fabrication processes, which reduce the yield to a great degree. Accordingly, the yield management tools are used to inspect and/or review the defects and the particles. To meet the more and more advanced requirements on performance of IC chips, the patterns with smaller and smaller critical feature dimensions have been adopted. Consequently, the conventional yield management tools with optical beam gradually become incompetent due to diffraction effect, and the yield management tools with electron beam are more and more employed. Compared to a photon beam, an electron beam has a shorter wavelength and thereby possibly offering superior spatial resolution. Currently, the yield management tools with electron beams employ the principle of scanning electron microscope (SEM) with a single electron beam, and as well known their throughputs are not competent for mass production. Although increasing the beam currents can improve the throughputs, the superior spatial resolutions will be fundamentally deteriorated by Coulomb Effect which increases with the beam currents.
For mitigating the limitation on throughput, instead of using a single electron beam with a large current, a promising solution is to use a plurality of electron beams each with a small current. The plurality of electron beams forms a plurality of probe spots or simply called as a probe spot array on one being-inspected or observed surface of a sample. The plurality of probe spots can respectively and simultaneously scan a plurality of small scanned regions within a large observed area on the sample surface. The electrons of each probe spot generate secondary electrons from the sample surface where they land on. The secondary electrons comprise slow secondary electrons (energies ≤50 eV) and backscattered electrons (energies close to landing energies of the electrons). The secondary electrons from the plurality of small scanned regions can be respectively and simultaneously collected by a plurality of electron detectors. Consequently, the image of the large observed area including all of the small scanned regions can be obtained much faster than scanning the large observed area with a single beam.
The plurality of electron beams can be either from a plurality of electron sources respectively, or from a single electron source. For the former, the plurality of electron beams is usually focused onto and scans the plurality of small scanned regions by a plurality of columns respectively, and the secondary electrons from each scanned region are detected by one electron detector inside the corresponding column. The apparatus therefore is generally called as a multi-column apparatus. On the sample surface, the beam interval or pitch is on the order of several to tens millimeters.
For the latter, a source-conversion unit virtually changes the single electron source into a plurality of sub-sources. The source-conversion unit comprises one beamlet-limit (or beamlet-forming) means with a plurality of beam-limit openings and one image-forming means with a plurality of electron optics elements. The plurality of beam-limit openings divides the primary-electron beam generated by the single electron source into a plurality of sub-beams or beamlets respectively, and the plurality of electron optics elements influence the plurality of beamlets to form a plurality of first parallel (virtual or real) images of the single electron source respectively. Each first image is the crossover of one beamlet and can be taken as one sub-source which emits the corresponding beamlet. To make more beamlets available, the beamlet intervals are at micro meter level. Naturally, one primary projection imaging system and one deflection scanning unit within one single column are used to project the plurality of first parallel images onto and scan the plurality of small scanned regions respectively. The plurality of secondary electron beams therefrom is directed by one beam separator into one secondary projection imaging system, and then focused by the secondary projection imaging system to be respectively detected by a plurality of detection elements of one electron detection device inside the single column. The plurality of detection elements can be a plurality of electron detectors placed side by side or a plurality of pixels of one electron detector. The apparatus therefore is generally called as a multi-beam apparatus
The beamlet-limit means is usually an electric-conduction plate with through-holes, and a plurality of through-holes therein functions the plurality of beam-limit openings respectively. For the image-forming means, each electron optics element either focuses one beamlet to form one real image (such as U.S. Pat. No. 7,244,949 and the fourth related application in the CROSS REFERENCE), or deflects one beamlet to form one virtual image (such as U.S. Pat. No. 6,943,349 and the other related applications in the CROSS REFERENCE). FIG. 1A and FIG. 1B show two examples in the fifth related application. For sake of clarity, only three beamlets are shown, and the deflection scanning unit, the beam separator, the secondary projection imaging system and the electron detection device are not shown.
In FIG. 1A, the primary-electron beam 102 generated by the electron source 101 is focused by the condenser lens 110 to be incident onto the source-conversion unit 120. The source-conversion unit 120 comprises one pre-beamlet-bending means 123 with three pre-bending micro-deflectors 123_1, 123_2 and 123_3, one beamlet-limit means 121 with three beam-limit openings 121_1, 121_2 and 121_3 and one image-forming means 122 with three electron optics elements 122_1, 122_2 and 122_3. The three pre-bending micro-deflectors 123_1˜123_3 respectively deflect the three beamlets 102_1˜102_3 perpendicularly incident onto the three beam-limit openings 121_1˜121_3, and each of which functions as a beam-limit aperture to limit the current of the corresponding beamlet. The three electron optics elements 122_1˜122_3 deflects the three beamlets 102_1˜102_3 towards the primary optical axis 100_1 and form three first virtual images of the electron source 101, i.e. each beamlet has a virtual crossover. The objective lens constitutes the primary projection imaging system, which focuses the three deflected beamlets 102_1˜102_3 onto the surface 7 of the sample 8, i.e. projecting the three first virtual images thereon. The three beamlets 102_1˜102_3 therefore form three probe spots 102_1s, 102_2s and 102_3s on the surface 7. The currents of the probe spots 102_1s˜102_3s can be varied by adjusting the focusing power of the condenser lens 110. In FIG. 1B, the movable condenser lens 210 focuses the primary-electron beam 102 to be normally incident onto the beamlet-limit means 121 of the source-conversion unit 220, and the pre-beamlet-bending means 123 in FIG. 1A is therefore not needed. Accordingly the currents of the probe spots 102_1s˜102_3s can be varied by adjusting the focusing power and the position of the movable condenser lens 210. In FIG. 1A and FIG. 1B, the landing energies of the beamlets 102_1˜102_3 on the sample surface 7 can be varied by adjusted either or both of the potentials of the electron source 101 and the sample surface 7.
In a multi-beam apparatus, each beamlet scans one sub-FOV (field of view) on the sample surface, and the total FOV is the sum of the sub-FOVs of the plural beamlets. Each sub-FOV is equal or smaller than the beamlet pitch on the sample surface (Ps in FIG. 1A). To further improve the throughput, each sub-FOV is better selectable in terms of the imaging resolution and the pitches of the plural beamlets are accordingly varied to keep the sub-FOVs stitched up. In one case with high image resolution, a small pixel size will be used and a small sub-FOV is desired for avoiding a large pixel number. In another case with low image resolution, a large pixel size will be used and a large sub-FOV is desired for a high throughput. FIG. 2A shows an example in the later case. As shown in dash line, if the probe spots 102_2s and 102_3s in FIG. 1A can be intentionally moved to the right and the left respectively, i.e. the pitch Ps can be changed from P1 to P2, the total FOV will be increased from 3×P1 to 3×P2, and accordingly the throughput is increased. Hence making the beamlet pitch Ps selectable will be one preferred function.
The continuous scanning mode (a sample continuously moving in the direction perpendicular to a scanning direction of a primary electron beam) is a conventional method to get high throughput in a conventional single-beam apparatus. If using this method in a multi-beam apparatus, it is better to match the orientation of the total FOV or the probe spot array with the stage moving direction. As well known, if there is one magnetic lens in the primary projection imaging system, the magnetic field thereof will rotate the plural beamlets and the total FOV as a result. Due to the magnetic field is varied with respect to the observing conditions (such as landing energies and currents of plural beamlets), the rotation angle of the total FOV will accordingly vary. FIG. 2B shows an example if the objective lens 131 in FIG. 1A is a magnetic or electromagnetic compound lens. For instance, when the landing energies of the beamlets 102_1˜102_3 are changed from 1 keV to 2 keV, the probe spots 102_2s and 102_3s will rotate an angle β around the optical axis 100_1 as shown in dash line, i.e. the orientation of the total FOV rotates the angle β. The orientation variation of the total FOV impacts the performance of the continuous scanning mode. Keeping the orientation of the probe spot array same or making it selectable can provide more flexibility to improve the throughput, and accordingly is another preferred function.
For some sample, a specific match between the orientations of patterns thereon and the probe spot array may be required. Making the orientation of the probe spot array selectable can compensate the mismatch due to the limited loading accuracy, and therefore can increase the throughput by avoiding the time-consuming of re-loading. In addition, to effectively observe some patterns of a sample, the plural beamlets may be required to land onto the sample surface with specific incident angles. Making the incident angles selectable can enable more samples or patterns observable, and will be one more preferred function.
The present invention will provide methods to realize the foregoing functions in a multi-beam apparatus, especially for those proposed in the CROSS REFERENCE and used as yield management tools in semiconductor manufacturing industry.